Accurate calibration of very long range radars or altimeters on board satellites is difficult to achieve using active targets because microwave delay lines having a duration of several milliseconds are not available for this purpose.
Ever since the invention of radar, calibration targets have been used for range calibration. Such passive targets constituted by metal reflectors must have a radar cross-section that is large enough to ensure that the power ratio of target echoes to clutter echoes from the ground and from the environment is sufficient for the desired calibration to be obtained accurately. Such targets are bulky, difficult to move, expensive, and they often depend on the polarization of the incident wave. That is why active calibration targets have been developed which are constituted by a receiver antenna and a transmitter antenna interconnected by an amplifier. They are smaller in size and the power of their echoes improves calibration accuracy. All such targets must be placed in the field of the radar and this cannot always be done, e.g. when using a long range radar or an altimeter radar on board a satellite or an aircraft. In addition, they are still subject to ground echoes. To be able to move the radar target closer, it is necessary to transform it into an echo simulator, i.e. to provide an active target with a delay line on the path followed by the microwave signals.
An article on this topic is "Delayed action reflector for external calibration of synthetic aperture radar" by K. Komiyama, published in Electronics Letters, 30 Mar. 1989, Vol. 25, No. 7, p. 468.
Acoustic or electronic microwave delay lines are not suitable for obtaining delays that are as large as is desired. That is why proposals have been made to use optical delay lines for calibration purposes, which delay lines are constituted by long delay-providing optical fibers. The low attenuation of an optical fiber makes it possible to achieve long delays, and the wide band width thereof makes it possible to carry the microwave signal directly on the lightwave without changing frequency.
It is also advantageous to observe that the delay achieved in this way is not sensitive to the type of radar pulse used because of the wide passband of an optical fiber.
The following articles deal with this topic:
"Wideband 150 .mu.s optical delay line for satellite altimetric radar checking" by M. Maignan, J. J. Bernard, P de Chateau Thierry, Electronics Letters, 7 Jul. 1988, Vol. 24, No. 14, p. 902; PA1 "Radar applications of X-band fiber optic links" by I. L. Newberg, C. M. Gee, G. D. Thurmond, 0149-645X/88/0000-0987$01.00 c 1988 IEEE; and PA1 "Long microwave delay fiber optic link for radar testing" by I. L. Newberg, G. D. Thurmond, W. Yen, IEEE Transactions on microwave theory and techniques, Vol. 38, No. 5, May 1990.